1. Field of the Invention
This invention relates to an image processing apparatus for processing color image data based on read color image of an original document, and more particularly to an improvement of an image processing apparatus in which black data is accurately generated to utilize the generated black data in processing the color image data.
2. Description of the Prior Art
Generally, electrophotographic digital color copiers are operated such that: photoelectric conversion elements such as CCDs receive light reflected from an illuminated document of a color image via a color separation filter of red (R), green (G), and blue (B) to separate color image data into light receiving signals of R, G, and B respectively indicative of light intensity of color components of red, green, and blue, an analog-to-digital (A/D) converter converts the light receiving signals into a digital value, and a density converter converts the digital values of color components of R, G, and B into color density data of cyan (C), magenta (M), and yellow (Y). Subsequently, black data k is generated based on the obtained color image data of C, M, Y, and under color removal (UCR) process is effected for the color image data of C, M, Y based on the generated black data k.
Black data generation and UCR process are effected to suppress consumption of color toners of C, M, and Y and to improve reproductivity of shadowed areas and character images, because in these processings, part of color image data of C, M, and Y is converted into black data (black data generation), and the level of color image data of C, M, and Y is lowered by the level corresponding to the thus obtained black data (under color removal).
The digital values of color image data of C, M, and Y are identical to one another means that the color image to be produced is supposed to be black. In other words, black data k is produced corresponding to the minimum value of C, M, Y, and is represented by the following equation: EQU k=min(C, M, Y)
It should be noted that the spectral transmittance of an actually used color separation filter of R, G, and B does not exhibit the ideal characteristic, and the actual spectral transmittance of the color separation filter partially overlies in the boundary between red and green, and green and blue. Accordingly, when a color image document is read by CCDs via a color separation filter, color image data of G becomes larger than the ideal one which should be visible to human eyes because the spectral transmittance of G partially overlies in the area of R and B.
Thereby, black data k obtained from the above equation, k=min(C,M,Y), becomes larger than the ideal value because of the general tendency of overlying character as described above. When such large black data k is utilized, there was a problem that green data or blue data, particularly green data that does not inherently contain black data is affected by the large black data k, thereby darkening the green color (undesirable mingle of black color).
To avoid such undesirable mingle of black color into green or blue, there has been proposed a technology in which the black data k is converted into an output value k' such that k' is set at 0 when the black data k is in the range from 0 to a certain value, and the output value k' is used as a parameter for UCR process.
Specifically, according to this conversion process, to completely shut out mingle of black color into green color, k' is set at 0 when 0.ltoreq.k.ltoreq.150 wherein the unit of k and k' is the gradation level ranging from 0 to 255. According to this process, the output level of black is suppressed to 0 for a relatively wide range from 0 to 150. As a result, black data generation is exceedingly suppressed for the wide range from 0 to 150. This does not contribute to the saving of consumption of color toners of C, M, and Y, and hinders improvement of reproductivity of shadowed areas and character images.
To eliminate the above drawback, there has been proposed an idea of setting the output value k' for black data in the following manner as shown in FIG. 6:
k'=0 when 0.ltoreq.k.ltoreq.60; and PA1 k'&gt;0 when k&gt;60.
According to this two-stage setting process, after k&gt;60, the output value k' gradually rises as the black data k increases. However, on the way of rise of gradation level, i.e., k.apprxeq.60, the output value k' abruptly rises from 0 to a certain level. At this time, one of color image data C, M, and Y has already reached the gradation level of 60 since min(C, M, Y).gtoreq.60. Accordingly, the amount of black toner rapidly increases around the gradation level of k.apprxeq.60, thereby resulting in an undesirable formation of an obscure black image (pseudo-contour-image). Thus, this two-stage setting process is not effective in completely preventing mingle of black color into other colors, and impairs reproductivity of copied image.